Crossflow type filtering operation method using ceramic filter

ABSTRACT

Provided is a simple filtering operation method capable of conducting a filtering operation without clogging in a ceramic filter for a long period of time, in the operation of filtering fine crystals of an aromatic carboxylic acid in an oxidation reaction mother liquor obtained in a process of an aromatic carboxylic acid production by a cross-flow filtration using the ceramic filter. The present invention can be accomplished by conducting an operation for filtering the fine crystals and a back washing operation with a filtrate while maintaining a flowing circulation operation of the oxidation reaction mother liquor under predetermined conditions.

TECHNICAL FIELD

The present invention relates to a filtering operation method of aceramic filter used in solid-liquid separation for removing finecrystals contained in an oxidation reaction mother liquor obtained in aprocess for producing an aromatic carboxylic acid to obtain a clearfiltrate.

BACKGROUND ART

Aromatic carboxylic acids are produced by liquid-phase oxidationreaction of alkyl group-containing aromatic hydrocarbons. In thisreaction, a heavy metal catalyst such as cobalt and manganese, or thecatalyst to which a promoter such as a bromine compound and an aldehydeis further added is usually used in the presence of an acetic acidsolvent.

An aromatic carboxylic acid-containing slurry obtained from such aliquid-phase oxidation reaction is usually subjected to crystallizationto reduce its temperature and then subjected to solid-liquid separationunder a pressure close to normal pressures to thereby obtain a cake ofan aromatic carboxylic acid.

On the other hand, an oxidation reaction mother liquor obtained in thesolid-liquid separation contains useful catalyst components derived fromthe catalyst such as heavy metal ions and bromide ions. Whenindustrially practicing the above reaction, it is necessary to recycleand reuse these catalyst components and thereby reduce production costs.

The simplest recycling method is a method in which the oxidationreaction mother liquor is fed as itself back to a reaction system andreused therein (mother liquor recycling), and this method has beenextensively used in commercial production processes. However, theoxidation reaction mother liquor contains various organic impuritiesby-produced in the liquid-phase oxidation reaction or inorganicimpurities produced owing to corrosion of an apparatus used. If theoxidation reaction mother liquor is reused as itself in the reactionsystem, the concentration of these impurities in the reaction systemtends to be gradually increased. As a result, it has been confirmed thatwhen the concentration of the impurities exceeds a predetermined level,the liquid-phase oxidation reaction tends to be adversely affected.

For example, it has been reported that if the aromatic carboxylic acidis isophthalic acid, the proportion of the oxidation reaction motherliquor fed back to the reaction system (mother liquor recycling rate) isusually from 60 to 90%. The remaining oxidation reaction mother liquorwhich does not serve for reuse in the reaction system and is present inan amount of from 10 to 40% is fed to a step of recovering acetic acidas a solvent (the mother liquor not fed back to the reaction system iscalled a “purge mother liquor”). Also, it has been reported if thearomatic carboxylic acid is 2,6-naphthalenedicarboxylic acid, theproportion of the oxidation reaction mother liquor fed back to thereaction system is usually from 30 to 90%. The remaining oxidationreaction mother liquor which does not serve for reuse in the reactionsystem and is present in an amount of from 10 to 70% is fed to a step ofrecovering acetic acid as a solvent.

As a method of recovering the catalyst components from the oxidationreaction mother liquor fed to the acetic acid recovering step andreusing it, there have been proposed the method using an anion-exchangeresin (Patent Documents 1 to 4) and the method using a pyridinering-containing chelate resin (refer to Patent Documents 5 and 6).

In Patent Document 2, it is described that an oxidation reaction motherliquor contains fine aromatic carboxylic acid crystals leaked from anaromatic carboxylic acid slurry upon solid-liquid separation thereof orprecipitated by temperature drop of the oxidation reaction motherliquor, and therefore when continuously feeding the oxidation reactionmother liquor to a resin column to contact with a resin, it is necessaryto remove the fine crystals using a filter, etc., in order to preventdeposition of the fine crystals on an upper portion or an inside portionof a resin layer. However, Patent Document 2 fails to describe aspecific method for removing the fine crystals. In Patent Document 3, itis described that solids need to be removed by filtering the oxidationreaction mother liquor in advance, at a temperature not higher than atemperature of the operation. Although an installation place of ahigh-grade filter for removing the fine crystals is specified in aschematic flow-diagram of the process, details of the filter itself isnot described. In Patent Documents 5 and 6, it is described that anoxidation reaction slurry is preferably subjected to solid-liquidseparation such that a content of crystals in the oxidation reactionmother liquor is 0.1% or less, but a filter for removing the finecrystals is not described.

In Patent Document 4, it is described that a mother liquor purge flow isfiltered through a filter medium to recover and recycle an insolublearomatic carboxylic acid and the other insoluble components. As examplesof the filter medium, there are mentioned a microfiltration filtermedium, an ultrafiltration filter medium, a membrane filter medium, across-flow filter medium, a hydro-cyclone filter medium, a cross-flowceramic microfiltration filter medium, a bag filter medium, a sinteredmetal cross-flow ceramic microfiltration filter medium, a cross-flowmicrofiltration filter medium or the like. It is also described thatamong these filter media the cross-flow filtration using ananticorrosive and high temperature-resistant ceramic filter is preferredbecause the aromatic carboxylic acid as a product material trapped onthe filter medium can be continuously removed and recovered. However,details of the filtering operation are not disclosed, and it is merelydescribed that a suitable turbulence is obtained when a Reynolds numberof a fluid entering into a flow path of the ceramic filter is largerthan about 13,000.

When the an oxidation reaction mother liquor containing fine crystals isprocessed by a cross-flow filtration method using a ceramic filter, thefine crystals deposited on a filtering membrane of the ceramic filterare filtered while always washing out with a circulating fluid flowingthrough a flow path to obtain a clear filtrate. However, since afiltering performance is gradually deteriorated owing to deposition ofthe fine crystals, it is required to interrupt the filtering operationto clean the ceramic filter.

As the method of the cleaning, there may be used a method in which thecleaning is conducted by interrupting circulation of the oxidationreaction mother liquor flowing through the flow path of the ceramicfilter, or a method in which it is conducted while continuing thecirculation thereof.

In the method in which the circulation is interrupted, in order toremove the deposited fine crystals, a wash solvent capable of dissolvingthe fine crystals (specifically, for example, an acetic acid solvent)flows through the flow path of the ceramic filter to clean a surface ofthe filtering membrane, and the wash solvent flows from the flow pathside to the filtration side to penetrate through the filtering membrane(normal washing with the wash solvent). In addition, the wash solvent iseffectively made to penetrate from the filtration side to the flow pathside of the ceramic filter (back washing with the wash solvent). Thus,by using the method in which the circulation is interrupted, it ispossible to fully clean the ceramic filter and completely restore afiltering performance thereof (refer to Patent Documents 7 and 8).However, the method in which the circulation is interrupted has variousproblems such as need of using a large amount of the wash solvent,time-consuming procedure, complicated valve operations upon feed andinterruption of the oxidation reaction mother liquor as well as uponfeed and interruption of the wash solvent, occurrence of treatment of alarge amount of the used wash solvent, and so on.

In the method of cleaning the ceramic filter in which the circulation ofthe oxidation reaction mother liquor is not interrupted, a filtrate isallowed to penetrate from the filtration side to the flow path side ofthe ceramic filter (back washing with the filtrate). This methodrequires no fresh wash solvent (producing no used wash solvent) and canbe easily conducted only by interrupting the filtering operation, sothat a filtering performance of the ceramic filter can be substantiallyrestored. Therefore, the above method has been frequently employed infiltering operations by a ceramic filter in a general cross-flow type(refer to Patent Documents 9 and 10).

In Patent Document 9, it is described that while circulating a stocksolution, a pressure higher than that on the stock solution side isinstantaneously applied onto a filtrate in a filter container by apiston, and the filtrate is forced into the filter at a linear velocitylarger than the filtering linear velocity to conduct the back washingoperation of the filter. This method is characterized in that the backwashing time is very short.

From the contents of Examples in this document, it is construed that thefeature of the method resides in that when the filtering linear velocityis lowered owing to clogging of the filter, the back washing operationwith the filtrate is conducted at a linear velocity larger than such alowered filtering linear velocity.

However, the above method has posed problems such as use of the pistonfacility when industrially practiced. Further the differential pressureapplied upon the back washing operation is not described.

In claim 7 of Patent Document 10, it is described that a filtrate isreversely flowed from an outer periphery side to a flow path side of afilter to remove particles fixed on a surface of a filtering membrane.In addition, in the specification, it is described that upon conductingthe cross-flow filtration, a periodic back washing is preferablyperformed in order to prevent fixing of a cake layer on the surface ofthe filtering membrane. There is mentioned such a back washing mechanismthat a sump for back washing (back washing pot) is provided on arecovery side of the filtrate, and the filtrate is reversely flowedtoward the flow path side by an air compressor or a pump. Further, inExample, as shown in FIG. 1, there is illustrated a structure includingthe air compressor and the back washing pot, which is capable ofreversely flowing the filtrate from the outer periphery side of thefilter to the flow path side thereof.

However, any of these Patent Documents merely describe a general backwashing, but fail to describe requirements for efficiently conductingthe back washing operation. Also, as the back washing with the filtrateis repeated, a recovery rate of a filtering performance is graduallylowered so that the filter finally fails to exhibit a desired filteringperformance. In such a case, after the circulation of the oxidationreaction mother liquor is interrupted, the ceramic filter should becleaned by normal washing with the wash solvent and/or back washing withthe wash solvent.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP 53-102290A-   Patent Document 2: JP 10-15390A-   Patent Document 3: JP 2002-12573A-   Patent Document 4: JP 2003-507160A-   Patent Document 5: WO 2008/072561A-   Patent Document 6: WO 2008/075572A-   Patent Document 7: JP 3-131312A-   Patent Document 8: JP 5-317024A-   Patent Document 9: JP 63-51913A-   Patent Document 10: JP 2000-140842A

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

Thus, in the operation for filtering fine aromatic carboxylic acidcrystals contained in an oxidation reaction mother liquor by across-flow filtration using a ceramic filter in an aromatic carboxylicacid production process, there is still present such a problem that thefollowing filtering operation methods have not been found yet:

(1) a simple filtering operation method in which the filtering operationwith the ceramic filter can be conducted by the cross-flow filtrationfor a long period of time without clogging; and

(2) a filtering operation method in which no use of a large amount of awash solvent is required.

Means for Solving the Problem

As a result of extensive and intensive researches for achieving theabove object, the present inventors have found a stable and simplefiltering operation method and have accomplished the present invention.

That is, the present invention relates to the following aspects (1) to(8).

(1) A filtering operation method for filtering fine crystals containedin an oxidation reaction mother liquor obtained in a process forproducing an aromatic carboxylic acid except for terephthalic acid by across-flow filtration using a ceramic filter while conducting a flowingcirculation operation of the oxidation reaction mother liquor,

the method comprising:

conducting (I) an operation for filtering the fine crystals; and

conducting (II) a back washing operation with a filtrate whilemaintaining the flowing circulation operation of the oxidation reactionmother liquor, (II) the back washing operation with the filtrate beingconducted under the following conditions:

(II-A) an operation time thereof that is in the range of from 5 to 60[s];

(II-B) a differential pressure between a filtration side and a flow pathside of the ceramic filter which is in the range of from 0.10 to 1.0[MPa];

(II-C) a feeding linear velocity of the filtrate that is in the range offrom 1.0 to 20 [m/h]; and

(II-D) a temperature of the filtrate that is in the range of from “atemperature of the oxidation reaction mother liquor” to “a temperaturehigher by 35° C. than that of the oxidation reaction mother liquor”.

(2) The filtering operation method as described in the above aspect (1),wherein (I) the operation for filtering the fine crystals is carried outunder the following conditions:

(I-A) an operation time which is in the range of from 60 to 1800 [s];

(I-B) a differential pressure between the flow path side and thefiltration side that is in the range of from 0.05 to 0.5 [MPa];

(I-C) a circulating linear velocity of the oxidation reaction motherliquor in the flow path of the ceramic filter that is in the range offrom 1000 to 10000 [m/h] as measured at an inlet of the flow path; and

(I-D) a filtering linear velocity of the filtrate that is in the rangeof from 0.5 to 3.0 [m/h].

(3) The filtering operation method as described in the above aspect (1)or (2), further comprising: conducting (III) a back washing operationwith a wash solvent while maintaining the flowing circulation operationof the oxidation reaction mother liquor.

(4) The filtering operation method as described in the above aspect (3),wherein (III) the back washing operation with the wash solvent isconducted under the following conditions:

(III-A) an operation time thereof that is in the range of from 5 to 120[s];

(III-B) a differential pressure between the filtration side and the flowpath side upon the back washing operation with the wash solvent that isin the range of from 0.10 to 1.0 [MPa];

(III-C) a feeding linear velocity of the wash solvent upon the backwashing operation that is in the range of from 1.0 to 20 [m/h]; and

(III-D) a temperature of the wash solvent that is in the range of from“the temperature of the oxidation reaction mother liquor” to “thetemperature higher by 35° C. than that of the oxidation reaction motherliquor”.

(5) The filtering operation method as described in the above aspect (3)or (4), wherein an operation comprising (I) the operation for filteringthe fine crystals and (II) the back washing operation with the filtrateis repeated, and

when a flow rate of the filtering operation is not restored by (II) theback washing operation with the filtrate, (III) the back washingoperation with the wash solvent is conducted.

(6) The filtering operation method as described in any one of the aboveaspects (3) to (5), wherein the wash solvent is acetic acid having awater content of from 0.1 to 30% by mass.

(7) The filtering operation method as described in any one of the aboveaspects (1) to (6), wherein upon the back washing operation with thefiltrate, a pressure on a circulation outlet conduit side of the ceramicfilter is reduced to produce a differential pressure thereby feeding thefiltrate.(8) The filtering operation method as described in any one of the aboveaspects (1) to (7), wherein the aromatic carboxylic aid comprises atleast one of benzoic acid, phthalic acid, isophthalic acid, m-toluicacid, trimesic acid, 3,5-dimethyl benzoic acid, trimellitic acid,pyromellitic acid, 1,5-naphthalenedicarboxylic acid and2,6-naphthalenedicarboxylic acid.

Effect of the Invention

It has been found that when fine crystals contained in an oxidationreaction mother liquor in an aromatic carboxylic acid production processare subjected to the cross-flow filtration using a ceramic filter, dueto the operation under limited conditions, it is possible to attain thefollowing effects:

(1) a filtering operation can be simply performed for a long period oftime without clogging of the ceramic filter;

(2) no use of a large amount of a wash solvent is needed; and

(3) an aromatic carboxylic acid contained in the wash solvent which hasbeen conventionally discarded together with the wash solvent uponcleaning can be recovered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pipeline diagram showing an embodiment of the presentinvention.

FIG. 2 is a pipeline diagram showing another embodiment of the presentinvention.

FIG. 3 is a sectional view of a ceramic filter shown in FIG. 1 or FIG.2.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

The aromatic carboxylic acid as described in the present invention is acompound produced by liquid-phase oxidation of an alkyl group-containingaromatic hydrocarbon. The alkyl group-containing aromatic hydrocarbonmay be a compound having an aromatic ring to which at least one methylsubstituent group is bonded, and the aromatic ring may be either anaromatic hydrocarbon ring or an aromatic heterocyclic ring. Specificexamples of the alkyl group-containing aromatic hydrocarbon includetoluene, o-xylene, m-xylene, 1,3,5-trimethyl benzene, 1,2,4-trimethylbenzene, 1,2,4,5-tetramethyl benzene, 2,4-dimethyl benzaldehyde,3,4-dimethyl benzaldehyde, 2,4,5-trimethyl benzaldehyde, 1,5-dimethylnaphthalene 2,6-dimethyl naphthalene, and so on.

Specific examples of the aromatic carboxylic acid obtained byliquid-phase oxidation of the alkyl group-containing aromatichydrocarbon include benzoic acid, phthalic acid, isophthalic acid,m-toluic acid, trimesic acid, 3,5-dimethyl benzoic acid, trimelliticacid, pyromellitic acid, 1,5-naphthalenedicarboxylic acid and2,6-naphthalenedicarboxylic acid, and so on. However, terephthalic acidis excluded from the aromatic carboxylic acid.

In the liquid-phase oxidation reaction of the alkyl group-containingaromatic hydrocarbon, a heavy metal compound and a bromine compound areused as catalysts.

The heavy metal compound as the liquid-phase oxidation reaction catalystcontains at least one of a cobalt compound and a manganese compound, andif required a nickel compound, a cerium compound, a zirconium compound,and so on may be further added thereto. The cobalt compound, themanganese compound and the other heavy metal compounds may berespectively used, for example, in the form of an organic acid salt, ahydroxide, a halide, a carbonate or the like. Among these, especiallypreferred are an acetic acid salt and a bromide.

The concentration of the heavy metal compound is defined by aconcentration thereof in the oxidation reaction mother liquor, and notparticularly limited as long as it lies within the range capable ofaccelerating the liquid-phase oxidation reaction. For example, theconcentration of cobalt ions is usually 100 ppm or more and preferably200 ppm or more, and the upper limit thereof is 6000 ppm or less andpreferably 5000 ppm or less. Also, the concentration of manganese ionsis usually 100 ppm or more and preferably 150 ppm or more, and the upperlimit thereof is 3000 ppm or less and preferably 2500 ppm or less.

The bromine compound as the liquid-phase oxidation reaction catalyst maybe any bromine compound as long as it is capable of being dissolved inthe reaction system and generating bromide ions. Examples thereofinclude inorganic bromine compounds such as hydrogen bromide, sodiumbromide and cobalt bromide, and organic bromine compounds such asbromoacetic acid and tetrabromoethane. Especially, hydrogen bromide,cobalt bromide or manganese bromide is suitable.

The concentration of the bromine compound used is defined by aconcentration thereof in the oxidation reaction mother liquor, and notparticularly limited as long as it lies within the range capable ofaccelerating the liquid-phase oxidation reaction. For example, theconcentration of bromide ions is usually 300 ppm or more and preferably500 ppm or more, and the upper limit thereof is 7000 ppm or less andpreferably 6000 ppm or less.

The temperature of the liquid-phase oxidation reaction is preferably inrange from 120 to 230° C. and more preferably from 140 to 210° C. Whenthe reaction temperature is excessively low, a large amount ofintermediate reaction products tend to remain in the resulting slurry.When the reaction temperature is excessively high, the acetic acidhaving a water content of from 1 to 15% by mass as a solvent tends toexhibit a large combustion loss.

The pressure in a reactor used in the liquid-phase oxidation reaction isnot particularly limited as long as the reaction system is capable ofmaintaining a liquid-phase at the reaction temperature, and is usuallyfrom 0.1 to 3.0 [MPaG] and preferably from 0.3 to 1.8 [MPaG].

Examples of a molecular oxygen-containing gas that is used as anoxidizing agent in the liquid phase oxidation reaction include air,inert gas-diluted oxygen and oxygen-rich air, but in view of facilitiesand costs the use of air is usually preferred.

The oxidation reaction slurry containing crude aromatic carboxylic acidcrystals which is produced in an oxidation reactor used in theliquid-phase oxidation reaction is preferably fed to a next oxidationreactor connected in series thereto, and is further subjected to finaloxidation reaction with an oxygen-containing gas. After that, followingdropping pressure and cooling through crystallization vessels having oneor more stages which are connected in series if required, then theslurry is fed to the subsequent solid-liquid separation step.

An example of the liquid-phase oxidation reaction is as follows. Forexample, using an apparatus with a commercial scale, m-xylene issubjected to liquid-phase oxidation by air (reaction temperature: 200 [°C.]; reaction pressure: 1.6 [MPaG]) in the presence of cobalt acetate,manganese acetate and hydrobromic acid in hydrous acetic acid to obtaina crude isophthalic acid slurry (the concentration of isophthalic acid:33% by weight; the concentration of water in the hydrous acetic acidthat is a dispersing medium: 14% by weight), and the resulting slurry isthen introduced into crystallization vessels connected in series tosubject the slurry to sequential pressure drop.

Next, the oxidation reaction slurry is cooled to separate the aromaticcarboxylic acid crystals therefrom. In the solid-liquid separation step,the crude aromatic carboxylic acid slurry produced in the liquid-phaseoxidation reaction is separated into crude aromatic carboxylic acidcrystals and an oxidation reaction mother liquor using a solid-liquidseparator. The solid-liquid separation is usually carried out underatmospheric pressure and may also be carried out under applied pressure.The temperature in the solid-liquid separation is not particularlylimited, and is usually a temperature lower than a boiling point of asolvent as measured under atmospheric pressure, for example, in therange of from 50 to 115° C. The upper limit of the temperature in theseparation under applied pressure is 150° C. Examples of thesolid-liquid separator include a centrifugal separator, a centrifugalfilter, a vacuum filter, and so on.

The oxidation reaction mother liquor separated in the solid-liquidseparation step contains the fine crystals containing the aromaticcarboxylic acid as a main component, and therefore, is subjected to across-flow filtration using a ceramic filter to recover the finecrystals of aromatic carboxylic acid, thereby improving the output levelof the aromatic carboxylic acid. In addition, the resulting clearoxidation reaction mother liquor can be directly subjected to a catalystrecovery process using a resin.

A general ceramic filter, which may be a filter utilizing a ceramicporous body is used, for example, for removing suspended matters,bacteria, dusts, etc., in a liquid or gas in a wide range of fields suchas water treatment, waste gas treatment, and medical and food fieldsbecause it is excellent in physical strength, durability, corrosionresistance or the like.

In the ceramic filter, the ceramic porous body itself may be used as afiltering material; however, in general, in order to enhance both afiltering performance and a fluid penetration rate (i.e., processingcapacity), the ceramic porous body is used as a substrate (support), anda filtering membrane of a ceramic material is formed on a surface of thesubstrate.

For example, whereas the filtering membrane is configured to have anaverage pore diameter as small as from about 0.01 to about 1.0 μm toensure a filtering performance, the substrate is configured to have anaverage pore diameter as large as from about 1 μm to about severalhundreds of microns to reduce a flow resistance inside of the substrateand enhance a fluid penetration rate (i.e., processing capacity).

The substrate of the ceramic filter may be formed into various shapesaccording to the object of the filtration. In general, the substrate maybe frequently used in form of a tube having a single flow path or inform of a honeycomb (including a monolith) having a number of flow pathsextending in parallel with each other.

The filter in which the filtering membrane is formed on a surface of thetubular or honeycomb-shaped substrate, e.g., on an inner peripheralsurface of a flow path therein, is mounted in a housing such that anouter periphery side of the substrate is hermetically isolated from anend surface side of the substrate to which the flow path is opened, byan O-ring, etc., thereby providing a cross-flow filter.

It is preferable that the ceramic filter used in the present inventioncomprising a ceramic porous body as the substrate (a support member) isin the form of a honeycomb (including a monolith) having a large numberof flow paths extending in parallel with each other in the substrate,wherein the filtering membrane composed of a ceramic is formed on aninner peripheral surface of the flow paths. The average pore diameter ofthe filtering membrane is preferably from 0.1 to 5 μm. As the ceramicfilter, a commercially available ceramic filter may be used and theexamples include a ceramic membrane filter available from NGKInsulators, Ltd., a ceramic membrane filter available from Pall Corp.,and a ceramic membrane filter available from TAMI Industries, France.

Also, a plurality of the ceramic filters may be disposed in parallelwith each other according to the amount of throughput.

In the cross-flow type filter, a fluid to be treated such as gas andliquid is fed from one end surface side of the substrate into the flowpaths, and then the filtered fluid that penetrates through the filteringmembrane disposed on an inner peripheral surface of the flow paths isrecovered from the outer peripheral surface side of the substrate. Onthe other hand, the fluid to be treated which is not filtered can berecovered from the other end surface side of the substrate.

In the above cross-flow filtration, the time of the operation forfiltering the fine crystals is from 60 to 1800 seconds and preferablyfrom 120 to 1200 seconds.

When the time of the operation for filtering is 60 seconds or longer, achange-over operation of valves can be prevented from taking placefrequently so that a service life of the valves can be maintained. Whenthe time of the operation for filtering is 1800 seconds or shorter, asufficient average filtering flow rate can be maintained withoutreducing the filtering flow rate to 0.20 [m³/h] or less at the time oftermination in the filtrating. In addition, the filtering performancemay be readily restored by a back washing with the filtrate.

The circulating linear velocity of the oxidation reaction mother liquorin the flow path of the ceramic filter upon conducting the operation forfiltering the fine crystals is preferably in the range of from 1000 to10000 [m/h] at an inlet of the flow path. When the circulating linearvelocity is 1000 [m/h] or more, it is possible to attain a sufficienteffect of removing the fine crystals deposited on a surface of thefiltering membrane. On the other hand, when the circulating linearvelocity is 10000 [m/h] or less, the differential pressure between aninlet side and an outlet side of the ceramic filter is prevented fromincreasing excessively, so that a pump for circulating the oxidationreaction mother liquor is not required to have an excessively largecapacity (i.e., it is possible to suppress increase in costs for plantinvestment), and further there occurs no fear of damage to the ceramicfilter or O-ring. Meanwhile, when the ceramic filter has a plurality offlow paths or when a plurality of the ceramic filters are used together,the circulating linear velocity in the flow paths of the ceramicfilter(s) is defined by an average circulating linear velocity ascalculated based on a total sectional area of the whole flow paths.

The filtering linear velocity of the filtrate upon conducting theoperation for filtering the fine crystals is preferably in the range offrom 0.5 to 3.0 [m/h]. When the filtering linear velocity of thefiltrate is 0.5 [m/h] or more, it is possible to attain a sufficientfiltering performance. On the other hand, when the filtering linearvelocity of the filtrate is 3.0 [m/h] or less, there occurs a less fearof damage to the ceramic filter or O-ring.

The differential pressure between the flow path side and the filtrationside upon conducting the operation for filtering the fine crystals ispreferably in the range of from 0.05 to 0.5 [MPa].

When the differential pressure is 0.05 [MPa] or more, it is possible toattain a sufficient filtering performance. On the other hand, when thedifferential pressure is 0.5 [MPa] or less, there occurs a less fear ofdamage to the ceramic filter or O-ring.

In the above filtration, the fine crystals are gradually deposited on asurface of the filtering membrane so that the filtering flow ratetherethrough is lowered. Therefore, while maintaining the flowingcirculation operation of the oxidation reaction mother liquor, thefiltering operation is changed-over to a back washing with the filtrate.This means an operation of feeding the filtrate from the filtration sideto the flow path side, and aims at physically and chemically removingthe fine crystals deposited on the surface of the filtering membrane.

In the back washing with the filtrate, the differential pressure betweenthe filtration side and the flow path side is in the range of from 0.10to 1.0 [MPa] and more preferably from 0.10 to 0.60 [MPa].

When the differential pressure between the filtration side and the flowpath side is less than 0.10 [MPa], it may be difficult for the filtrateto uniformly flow through a whole portion of the filtering membrane ofthe ceramic filter, so that no sufficient washing effect can beobtained. On the other hand, when the differential pressure between thefiltration side and the flow path side is more than 1.0 [MPa], it isundesirable since there tends to occur rapid change in pressure so thata possibility of damage to the ceramic filter or seal members thereforis increased.

The feeding linear velocity of the filtrate is in the range of from 1.0to 20 [m/h], preferably from 1.0 to 15 [m/h], more preferably from 1.0to 10 [m/h] and still more preferably from 1.0 to 8 [m/h].

When the feeding linear velocity of the filtrate is less than 1.0 [m/h],it may be difficult for the filtrate to uniformly flow through a wholeportion of the filtering membrane of the ceramic filter, so that nosufficient washing effect can be obtained. On the other hand, when thefeeding linear velocity of the filtrate is more than 20 [m/h], it isundesirable, since there tends to occur rapid change in pressure so thata possibility of damage to the ceramic filter or seal members thereforis increased.

Upon the back washing with the filtrate, it is suitable that thepressure on the flow path side is lowered to produce the differentialpressure.

On the flow path side, the oxidation reaction mother liquor iscirculated, and in order to regulate the circulating flow rate thereof,there are used control valves disposed on an upstream of a filteringmodule (on a conduit on the side of feeding the oxidation reactionmother liquor to the filtering module) and on a downstream thereof (on aconduit on the side of feeding the oxidation reaction mother liquorwhich passes through the flow paths of the ceramic filter back to anoxidation reaction mother liquor vessel from the filtering module). Thecontrol valves on the upstream and downstream control not only thecirculating flow rate but also a pressure on the flow path side. Byusing these control valves, it is possible to reduce the pressure on theflow path side. In particular, upon the back washing with the filtrate,the filtrate used in the back washing is merged with the oxidationreaction mother liquor circulated through the flow path, so that theflow rate is increased on the downstream of the filtering module;therefore, it is preferred that an opening degree of the control valveon the downstream is increased, and it is preferred that at the sametime an opening degree of the control valve on the upstream is decreasedto suppress variation in circulating flow rate on the upstream owing toentrance of the filtrate used for the back washing operation into theflow path. Due to these, it is possible to reduce the pressure in theflow paths as a whole.

In order to remove the fine crystals deposited on the surface of thefiltering membrane and solids precipitated inside of the filteringmembrane (mainly the aromatic carboxylic acid), it is advantageous thatthe feeding time of the filtrate upon the back washing is longer.However, in view of a loss of the filtering operation time, it isrequired that it lies within a certain range. Therefore, the time of theback washing operation with the filtrate is preferably in the range offrom 5 to 60 [s] and more preferably from 10 to 40 [s].

The filtrate used in the back washing is mixed with the oxidationreaction mother liquor circulated through the flow path, and therefore,the temperature of the filtrate is preferably equal to or more than thetemperature of the oxidation reaction mother liquor. In addition, inorder to dissolve the fine crystals deposited on the surface of thefiltering membrane and the solids precipitated inside of the filteringmembrane (mainly aromatic carboxylic acid) for a short period of time,it is preferred that the temperature of the filtrate is higher than thatof the oxidation reaction mother liquor. On the other hand, there is apossibility that the excessively large change in temperature in theceramic filter causes the damage to the ceramic filter or the O-ringowing to the difference in thermal expansion coefficient between theceramic filter and the filter housing. Therefore, the upper limit of thetemperature of the filtrate used in the back washing is preferably “atemperature higher by 35° C. than that of the oxidation reaction motherliquor” and is more preferably “a temperature higher by 25° C. than thatof the oxidation reaction mother liquor”.

In the present invention, (I) the operation for filtering the finecrystals; and (II) the back washing operation with the filtrate whilemaintaining the flowing circulation operation of the oxidation reactionmother liquor are alternately repeated.

As the back washing with the filtrate is repeated, the recovery rate ofthe filtering performance is gradually reduced, and finally it is notpossible to attain a predetermined filtering performance. In the presentinvention, in such a case, the ceramic filter is cleaned by a backwashing with a wash solvent while maintaining the flowing circulationoperation of the oxidation reaction mother liquor.

In the present invention, when conducting the back washing operationwith the wash solvent while maintaining the flowing circulationoperation of the oxidation reaction mother liquor, the wash solvent usedtherein is required to have a capability of dissolving the fine crystalscontaining the aromatic carboxylic acid as a main component, and aceticacid, which is used as the solvent in the liquid-phase oxidationreaction, is suitable as the wash solvent. The acetic acid preferablyhas a water content of from 0.1 to 30% by mass and more preferably from0.1 to 10% by mass.

When feeding the wash solvent from the filtration side to the flow pathside (upon the back washing with the wash solvent), in order tophysically and chemically remove the fine crystals deposited on thesurface of the filtering membrane, the differential pressure between thefiltration side and the flow path side is preferably in the range offrom 0.10 to 1.0 [MPa] and more preferably from 0.10 to 0.8 [MPa].

When the differential pressure between the filtration side and the flowpath side is 0.10 [MPa] or more, it is possible to allow the washsolvent to uniformly flow through a whole portion of the filteringmembrane of the ceramic filter and therefore attain a sufficient washingeffect. On the other hand, when the differential pressure between thefiltration side and the flow path side is 1.0 [MPa] or less, a rapidvariation in pressure does not occur, and there is no possibility ofdamage to the ceramic filter or seal members therefor.

The feeding linear velocity of the wash solvent is preferably in therange of from 1.0 to 20 [m/h] and more preferably from 1.0 to 15 [m/h].

When the feeding linear velocity of the wash solvent is 1.0 [m/h] ormore, it is possible to allow the wash solvent to uniformly flow througha whole portion of the filtering membrane of the ceramic filter andtherefore attain a sufficient washing effect. On the other hand, whenthe feeding linear velocity of the wash solvent is 20 [m/h] or less,there is no possibility of damage to the ceramic filter or seal memberstherefor by the rapid variation in pressure.

In order to remove the fine crystals deposited on the surface of thefiltering membrane and solids precipitated inside of the filteringmembrane (mainly the aromatic carboxylic acid), it is advantageous thatthe feeding time of the wash solvent is longer. However, in view of aloss of the filtering operation time as well as addition of the usedwash solvent to the oxidation reaction mother liquor, it is requiredthat the feeding time of the wash solvent lies within a certain range.Therefore, the time of the back washing operation with the wash solventis preferably in the range of from 5 to 120 [s] and more preferably from5 to 90 [s].

The wash solvent is mixed with the oxidation reaction mother liquorcirculated through the flow path, and therefore, the temperature of thewash solvent is preferably equal to or more than a temperature of theoxidation reaction mother liquor. In addition, in order to dissolve thefine crystals deposited on the surface of the filtering membrane and thesolids precipitated inside of the filtering membrane (mainly thearomatic carboxylic acid) for a short period of time, it is preferredthat the temperature of the wash solvent is higher than that of theoxidation reaction mother liquor. On the other hand, there ispossibility that the excessively large change in temperature in theceramic filter causes the damage to the ceramic filter or the O-ringowing to the difference in thermal expansion coefficient between theceramic filter and the filter housing. Therefore, the upper limit of thetemperature of the wash solvent is preferably “a temperature higher by35° C. than that of the oxidation reaction mother liquor” and is morepreferably “a temperature higher by 25° C. than that of the oxidationreaction mother liquor”.

In the filtering operation method according to the present inventionwhich is conducted by a cross-flow filtration using a ceramic filtercomprising: (I) the operation for filtering the fine crystals; (II) theback washing operation with the filtrate while maintaining the flowingcirculation operation of the oxidation reaction mother liquor; and (III)the back washing operation with the wash solvent while maintaining theflowing circulation operation of the oxidation reaction mother liquor,the order of the above operations (I), (II) and (III) is notparticularly limited, and these operations may be conducted in anyorder.

However, in the present invention, the method is preferable a periodicoperation comprising the operations (I), (II) and (III) in which theoperations (I) and (II) are repeated, and when the filtering flow rateis unable to be restored by the operation (II), the operation (III) iscarried out.

EXAMPLES

The present invention will be described in more detail below byreferring to the following Examples, etc. It should be noted, however,that the present invention is not limited by the following Examples.

In hydrous acetic acid having a water content of 9% by mass, m-xylenewas subjected to liquid-phase oxidation reaction by air (reactiontemperature: 200 [° C.]; reaction pressure: 1.6 [MPaG]) in the presenceof 1000 ppm of cobalt ions, 930 ppm of manganese ions and 950 ppm ofbromide ions to thereby obtain a slurry containing a crude isophthalicacid. After conducting post-oxidation reaction, the slurry wasintroduced into a crystallization step and was subjected to release ofpressure and cooled to 100° C. under a normal pressure, and thensubjected to solid-liquid separation using a rotary vacuum filter toseparate the crude isophthalic acid crystals from the reaction solution,thereby preparing an oxidation reaction mother liquor required for afiltering operation conducted by a cross-flow filtration using a ceramicfilter. Meanwhile, the oxidation reaction mother liquor was at 80° C.The oxidation reaction mother liquor was turbid owing to fine crystals,and the content of the fine crystals therein was 0.47% by mass.

A ceramic filter available from NGK Insulators, Ltd., as an element wasmounted in a filtering module 7. A ceramic filter 8 was in form of amonolith, a filtering membrane thereof had an average pore diameter of0.5 μm, and the size was of 30 mmφ×1000 mL. Flow paths 28 had an outerdiameter of 4 [mm], and the number of openings thereof was 19. Thefiltering area of the ceramic filter was 0.2386 [m²], and a totalsectional area of the flow paths was 0.0002386 [m²].

Meanwhile, the amount of discarded isophthalic acid in Examples wascalculated from the amount of precipitated isophthalic acid incirculating flow rate (0.68 m³/h) of the oxidation reaction motherliquor.

The operation method of cross-flow filtration using a ceramic filterwill be explained.

(Operation for Filtering Fine Crystals)

In FIG. 1 to FIG. 3, the above-mentioned oxidation reaction motherliquor is stored in an oxidation reaction mother liquor vessel 1. Theoxidation reaction mother liquor is fed to an upper substrate endsurface side 10 of a filtering module 7 through a circulation inletconduit 6 using a pump 2. At this time, the flow rate thereof isregulated by a control valve 4. The oxidation reaction mother liquor fedto the filtering module 7 passes through flow paths 28 in a ceramicfilter 8, and then passes through a lower substrate end surface side 10and a circulation outlet conduit 11, and is fed back to the oxidationreaction mother liquor vessel 1. At this time, a flow rate thereof isregulated by a control valve 13.

A series of the above flows is called an oxidation reaction motherliquor circulation line. In the mother liquor circulation line, bycontrolling the control valves 4 and 13, the circulating flow rate ofthe mother liquor is regulated. Further, a pressure on the side of theflow path 28 of the ceramic filter 8, namely, a pressure on an upstreamside of the filter (primary pressure), is controlled. The pressure isdetermined by pressure gauges 5 and 12.

The filtering operation of the oxidation reaction mother liquor isconducted by establishing the mother liquor circulation line and thenopening a valve 18 (while closing control valves 24 and 25). At thistime, the oxidation reaction mother liquor is filtered through theceramic filter 8, and thereby a clear oxidation reaction mother liquorfrom which the fine crystals are removed is discharged to a filtrationside 9, and then passes through a filtrate outlet conduit 16 and isstored in a filtrate vessel 19. A pressure on a downstream side of thefilter (secondary pressure) during the filtering operation is determinedby a pressure gauge 17.

The filtering flow rate is determined from a filtering differentialpressure between the primary pressure and the secondary pressure,properties of the fluid to be filtered (such as viscosity) and afiltering performance of the ceramic filter 8 (such as filtering area,average pore diameter and degree of clogging).

The filtering differential pressure ΔP [MPa] is calculated according tothe following formula:ΔP=(P1+P2)/2−P3wherein a pressure at the pressure gauge 5 is P1 [MPaG]; a pressure atthe pressure gauge 12 is P2 [MPaG]; and a pressure at the pressure gauge17 is P3 [MPaG].(Back Washing Operation with Filtrate)

In FIG. 1 to FIG. 3, when the filtering flow rate upon the operation forfiltering the fine crystals is lowered, a back washing operation withthe filtrate is conducted. First, the valve 18 is closed (while keepingthe control valves 24 and 25 in a closed state) to terminate theoperation for filtering (but while continuing circulation of the motherliquor). Next, the control valve 24 is opened while controlling, and thefiltrate in the filtrate vessel 19 is fed to the filtration side 9 ofthe filtering module 7 through a filtrate back washing conduit 23 usinga pump 20. At this time, if required, a temperature of the filtrate iscontrolled by a heat exchanger 22. The pressure on the filtration side 9is made to be higher than the pressure on the flow path side 28 (backwashing differential pressure), so that the filtrate passes from thefiltration side 9 to the flow path side 28 to conduct a back washing.The back washing differential pressure is defined by −ΔP.

Meanwhile, the control valves 4 and 13 may be appropriately controlledupon the back washing such that the pressure on the flow path side 28 islowered to adjust the back washing differential pressure.

(Back Washing Operation with Wash Solvent)

In FIG. 2, when the back washing operation with the filtrate fails tofully restore the filtering flow rate, a back washing operation with awash solvent is conducted. First, the valve 18 and the control valve 24are closed (while continuing circulation of the mother liquor). Next,the control valve 25 is opened while controlling to feed the washsolvent to the filtration side 9 of the filtering module 7 through awash solvent back washing conduit 26. At this time, if required, atemperature of the wash solvent is controlled by a heat exchanger 27.The pressure on the filtration side 9 is made to be higher than thepressure on the flow path side 28, so that the wash solvent passes fromthe filtration side 9 to the flow path 28 side to conduct the backwashing. The passing wash solvent is mixed with the oxidation reactionmother liquor circulated therethrough.

Meanwhile, the control valves 4 and 13 may be appropriately controlledupon the back washing such that the pressure on the flow path 28 side islowered to adjust the back washing differential pressure.

Further, an average filtering flow rate X₁ upon conducting the operationfor filtering the fine crystals; a circulating linear velocity LV₁ ofthe oxidation reaction mother liquor in the flow path of the ceramicfilter; and a filtering linear velocity LV₂ of the filtrate aredetermined according to the following formulae. Meanwhile, LV₁ isdetermined as an average circulating linear velocity.

Circulating flow rate at an inlet of filtering module 7: F [m³/h]

Time of the operation for filtering fine crystals: T₁ [s]

Filtering flow rate upon initiation of the operation for filtering finecrystals: X_(s) [m³/h]

Filtering flow rate upon termination of the operation for filtering finecrystals: X_(t) [m³/h]

Average filtering flow rate upon conducting the operation for filteringfine crystals:X ₁[m³/h]=(X _(s) +X _(t))/2Circulating linear velocity of oxidation reaction mother liquor in flowpath of ceramic filter:LV₁[m/h]=F/0.0002386Filtering linear velocity of filtrate:LV₂[m/h]=X _(s)/0.2386

In addition, an average filtering flow rate V₃ during one cycle of theoperation for filtering the fine crystals and the back washing operationwith the filtrate; and a feeding linear velocity LV₃ of the filtrateupon the back washing operation are determined according to thefollowing.

Total amount of filtrate in the operation for filtering fine crystals:V ₁[m³ ]=T ₁ ×X ₁/3600Time of back washing operation with filtrate: T₂ [s]Back washing flow rate upon back washing operation with filtrate: X₂[M³/h]Total amount of back washing fluid upon back washing operation withfiltrate:V ₂[m³ ]=T ₂ ×X ₂/3600Average filtering flow rate during one cycle of the operation forfiltering fine crystals and back washing operation with filtrate:V ₃[m³/h]=(V ₁ −V ₂)/(T ₁ +T ₂)×3600Feeding linear velocity upon back washing operation with filtrate:LV₃[m/h]=X ₂/0.2386

As the used amount of the filtrate upon the back washing operation withthe filtrate increases, a net amount of the obtained filtrate becomessmaller, and the average filtering flow rate V₃ during one cycle of theoperation for filtering the fine crystals and the back washing operationwith the filtrate is reduced.

Example 1 Operation for Filtering Fine Crystals

The above-mentioned oxidation reaction mother liquor (80° C.) was fed tothe filtering module 7 at a circulation inlet flow rate of 0.68 [m³/h](P1=0.10 [MPaG]) (down flow) to initiate a cross-flow filtering. Thefiltering flow rate X_(s) upon initiation of the operation for filteringthe fine crystals was 0.32 [m³/h] (P3=0.00 [MPaG]), and the circulationoutlet flow rate was 0.36 [m³/h] (P2=0.09 [MPaG]). The differentialpressure ΔP between the flow path side and the filtration side was 0.10[MPa]. Also, the circulating linear velocity in the flow paths and thefiltering linear velocity of the filtrate were 2848 (m/h) and 1.34(m/h), respectively.

(Back Washing Operation with Filtrate)

When continuing the operation for filtering, the filtering flow rate wasdecreased down to 0.20 [m³/h] after the elapse of 500 seconds (thefiltering flow rate X_(t) upon termination of the operation forfiltering the fine crystals was 0.20 [m³/h]). Therefore, whilecontinuing circulation of the oxidation reaction mother liquor, the backwashing operation with the filtrate was conducted for 15 seconds(feeding linear velocity: 2.85 [m/h]; temperature of filtrate: 80° C.).At this time, the respective pressures were P1=0.14 [MPaG], P2=0.10[MPaG] and P3=0.31 [MPaG], and the differential pressure −ΔP between thefiltration side and the flow path side was 0.19 [MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering. At this time, the filtering flowrate (immediately after the back washing with the filtrate in the 1stcycle=X_(s) in the 2nd cycle) was 0.32 [m³/h], and the average filteringflow rate V₃ in the 1st cycle of the operation for filtering the finecrystals and the back washing operation with the filtrate was 0.233[m³/h]. Also, the filtering flow rate immediately after the back washingwith the filtrate in the 2nd cycle was 0.32 [m³/h], and the averagefiltering flow rate V₃ in the 2nd cycle was 0.233 [m³/h].

The filtering flow rates immediately after the back washing with thefiltrate and the average filtering flow rates V₃ in the 3rd to 5thcycles were respectively the same as those in the 2nd cycle.

Further, the operation for filtering (for 500 seconds) and the backwashing operation with the filtrate (for 15 seconds) were alternatelyand continuously repeated. As a result, the ceramic filter was able tomaintain its filtering performance for filtering the fine crystals untilthe elapse of 6 hours when the filtering flow rate immediately after theback washing with the filtrate was unable to be restored to 0.25 [m³/h]or more. The results are shown in Table 1.

Example 2

The operation for filtering and the back washing operation with thefiltrate were alternately and continuously repeated in the same manneras in Example 1 except that the back washing operation with the filtratewas conducted at a feeding linear velocity of 5.70 [m/h] for 10 secondsas the time of the back washing operation. Upon conducting the backwashing operation with the filtrate, the respective pressures wereP1=0.16 [MPaG], P2=0.11 [MPaG] and P3=0.53 [MPaG], and the differentialpressure −ΔP upon the back washing between the filtration side and theflow path side was 0.40 [MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering. At this time, the filtering flowrate (immediately after the back washing with the filtrate in the 1stcycle) was 0.32 [m³/h], and the average filtering flow rate V₃ in the1st cycle was 0.228 [m³/h]. Also, the filtering flow rate immediatelyafter the back washing operation with the filtrate in the 2nd cycle was0.32 [m³/h], and the average filtering flow rate V₃ in the 2nd cycle was0.228 [m³/h].

The filtering flow rates immediately after the back washing with thefiltrate and the average filtering flow rates V₃ in the 3rd to 5thcycles were respectively the same as those in the 2nd cycle.

Further, the operation for filtering (for 500 seconds) and the backwashing operation with the filtrate (for 10 seconds) were alternatelyand continuously repeated. As a result, the ceramic filter was able tomaintain its performance for filtering the fine crystals until theelapse of 8 hours when the filtering flow rate immediately after theback washing with the filtrate was unable to be restored to 0.25 [m³/h]or more. The results are shown in Table 1.

Example 3

The operation for filtering and the back washing operation with thefiltrate were alternately and continuously repeated in the same manneras in Example 1 except that the back washing operation with the filtratewas conducted at a feeding linear velocity of 1.42 [m/h] with a filtratetemperature of 90° C. for 30 seconds as the time of the back washingoperation. Upon conducting the back washing operation with the filtrate,the respective pressures were P1=0.12 [MPaG], P2=0.10 [MPaG] and P3=0.21[MPaG], and the differential pressure −ΔP upon the back washing betweenthe filtration side and the flow path side was 0.10 [MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering. At this time, the filtering flowrate (immediately after the back washing with the filtrate in the 1stcycle) was 0.32 [m³/h], and the average filtering flow rate V₃ in the1st cycle was 0.226 [m³/h]. Also, the filtering flow rate immediatelyafter the back washing with the filtrate in the 2nd cycle was 0.32[m³/h], and the average filtering flow rate V₃ in the 2nd cycle was0.226 [m³/h].

The filtering flow rates immediately after the back washing with thefiltrate and the average filtering flow rates V₃ in the 3rd to 5thcycles were respectively the same as those in the 2nd cycle.

Further, the operation for filtering (for 500 seconds) and the backwashing operation with the filtrate (for 30 seconds) were alternatelyand continuously repeated. As a result, the ceramic filter was able tomaintain its performance for filtering the fine crystals until theelapse of 6 hours when the filtering flow rate immediately after theback washing with the filtrate was unable to be restored up to 0.25[m³/h] or more. The results are shown in Table 1.

Example 4

The operation for filtering and the back washing operation with thefiltrate were alternately and continuously repeated in the same manneras in Example 1 except that upon the back washing operation with thefiltrate, the opening degrees of the upstream control valve 4 and thedownstream control valve 13 for reducing the pressure on the flow pathside were not controlled. Upon conducting the back washing with thefiltrate, the respective pressures were P1=0.36 [MPaG], P2=0.31 [MPaG]and P3=0.51 [MPaG], and the differential pressure −ΔP upon the backwashing between the filtration side and the flow path side was 0.18[MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering. At this time, the filtering flowrate (immediately after the back washing with the filtrate in the 1stcycle) was restored to merely 0.31 [m³/h], and the average filteringflow rate V₃ in the 1st cycle was 0.233 [m³/h].

The filtering flow rate immediately after the back washing operationwith the filtrate in the 2nd cycle was restored to 0.31 [m³/h], and theaverage filtering flow rate V₃ in the 2nd cycle was 0.228 [m³/h].

The filtering flow rates immediately after the back washing with thefiltrate and the average filtering flow rates V₃ in the 3rd to 5thcycles were respectively the same as those in the 2nd cycle.

Further, the operation for filtering (for 500 seconds) and the backwashing operation with the filtrate (for 15 seconds) were alternatelyand continuously repeated. As a result, the ceramic filter was able tomaintain its performance for filtering the fine crystals until theelapse of 3 hours when the filtering flow rate immediately after theback washing with the filtrate was unable to be restored to 0.25 [m³/h]or more. The results are shown in Table 1.

Thus, since the opening degrees of the upstream control valve 4 and thedownstream control valve 13 for reducing the pressure on the flow pathside were not controlled, the filtrate flowing into the flow path uponthe back washing with the filtrate caused reduction in the flow rate ofthe oxidation reaction mother liquor entering into the ceramic filterfrom the upper substrate end surface side, thereby deteriorating theeffect of physically removing the fine crystals deposited on the surfaceof the filtering membrane. The results are also shown in Table 1.

Comparative Example 1

The operation for filtering and the back washing operation with thefiltrate were alternately and continuously repeated in the same manneras in Example 1 except that the back washing operation with the filtratewas conducted for 70 seconds. Upon conducting the back washing operationwith the filtrate, the respective pressures were P1=0.14 [MPaG], P2=0.10[MPaG] and P3=0.31 [MPaG], and the differential pressure −ΔP upon theback washing between the filtration side and the flow path side was 0.19[MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering. At this time, the filtering flowrate (immediately after the back washing with the filtrate in the 1stcycle) was 0.32 [m³/h], and the average filtering flow rate V₃ in the1st cycle was 0.145 [m³/h]. Also, the filtering flow rate immediatelyafter the back washing operation with the filtrate in the 2nd cycle was0.32 [m³/h], and the average filtering flow rate V₃ in the 2nd cycle was0.145 [m³/h].

The filtering flow rates immediately after the cycle back washing withthe filtrate and the average filtering flow rates V₃ in the 3rd to 5thcycles were respectively the same as those in the 2nd cycle.

Further, the operation for filtering (for 500 seconds) and the backwashing operation with the filtrate (for 70 seconds) were alternatelyand continuously repeated. As a result, the ceramic filter was able tomaintain its performance for filtering the fine crystals until theelapse of 6 hours when the filtering flow rate as measured immediatelyafter the back washing with the filtrate was unable to be restored to0.25 [m³/h] or more. The results are shown in Table 1.

However, since the back washing operation with the filtrate wasconducted for a prolonged time, the average filtering flow rate V₃ was0.145 [m³/h]. The low average filtering flow rate resulted in increasednumber of filter elements required, thereby causing increase in costs.The results are also shown in Table 1.

Comparative Example 2

The operation for filtering and the back washing operation with thefiltrate were alternately repeated in the same manner as in Example 1except that the back washing operation with the filtrate was conductedat a feeding linear velocity of 0.84 [m/h]. Upon conducting the backwashing operation with the filtrate, the respective pressures wereP1=0.11 [MPaG], P2=0.09 [MPaG] and P3=0.15 [MPaG], and the differentialpressure −ΔP upon the back washing between the filtration side and theflow path side was 0.05 [MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering, but the filtering flow rate uponinitiation of the operation for filtering the fine crystals was restoredto only 0.25 [m³/h]. Moreover the average filtering flow rate V₃ in the1st cycle of the operation for filtering the fine crystals and the backwashing operation with the filtrate was 0.247 [m³/h].

Next, the operation for filtering of the 2nd cycle was initiated.However, after the elapse of 60 s, the filtering flow rate was decreaseddown to 0.20 [m³/h] so that the time of the operation for filtering thefine crystals which was set to T₁=500 s was unable to be maintained. Theresults are shown in Table 1.

Comparative Example 3

The operation for filtering and the back washing operation with thefiltrate were alternately repeated in the same manner as in Example 1except that the back washing operation with the filtrate was conductedat a filtrate temperature of 70° C. Upon conducting the back washingoperation with the filtrate, the respective pressures were P1=0.14[MPaG], P2=0.10 [MPaG] and P3=0.34 [MPaG], and the differential pressure−ΔP upon the back washing between the filtration side and the flow pathside was 0.22 [MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering, but the filtering flow rate uponinitiation of the operation for filtering the fine crystals was restoredto only 0.24 [m³/h], and the average filtering flow rate V₃ in the 1stcycle was 0.233 [m³/h].

Next, the operation for filtering of the 2nd cycle was initiated.However, after the elapse of 60 seconds, the filtering flow rate wasdecreased down to 0.20 [m³/h] so that the setting in which the time ofthe operation for filtering the fine crystals T₁ was 500 seconds wasunable to be maintained. The results are shown in Table 1.

The reason therefor was that since the temperature of the filtrate uponthe back washing operation with the filtrate was as low as 70° C., itwas not possible to clean the ceramic filter to a sufficient extent.

Comparative Example 4

The operation for filtering and the back washing-operation with thefiltrate were alternately and continuously repeated in the same manneras in Example 1 except that the back washing operation with the filtratewas conducted at a feeding linear velocity of 0.84 [m/h] with a filtratetemperature of 90° C. the time of the back washing operation of 300seconds. Upon conducting the back washing operation with the filtrate,the respective pressures were P1=0.11 [MPaG], P2=0.09 [MPaG] and P3=0.15[MPaG], and the differential pressure −ΔP upon the back washing betweenthe filtration side and the flow path side was 0.05 [MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering. At this time, the filtering flowrate (immediately after the back washing with the filtrate in the 1stcycle) was 0.32 [m³/h], and the average filtering flow rate V₃ in the1st cycle was 0.088 [m³/h]. Also, the filtering flow rate immediatelyafter the back washing with the filtrate in the 2nd cycle was 0.32[m³/h], and the average filtering flow rate V₃ in the 2nd cycle was0.088 [m³/h].

The filtering flow rates immediately after the back washing with thefiltrate and the average filtering flow rates V₃ in the 3rd to 5thcycles were respectively the same as those in the 2nd cycle.

Further, the operation for filtering (for 500 seconds) and the backwashing operation with the filtrate (for 300 seconds) were alternatelyand continuously repeated. As a result, the ceramic filter was able tomaintain its performance for filtering the fine crystals until theelapse of 1.5 h when the filtering flow rate as measured immediatelyafter the back washing operation with the filtrate was unable to berestored up to 0.25 [m³/h] or more.

However, even under the conditions in this Comparative Example, theaverage filtering flow rate V₃ was 0.088 [m³/h]. The average filteringflow rate was low, which resulted in increase in number of filterelements required, thereby causing increase in costs. The results areshown in Table 1.

TABLE 1 Examples 1 2 3 4 (Operation conditions) Time of operation forfiltering fine crystals T₁ 500 500 500 500 [s] Time of back washingoperation with filtrate T₂ 15 10 30 15 [s] Feeding linear velocity offiltrate upon back 2.85 5.70 1.42 2.85 washing operation [m/h] Backwashing flow rate of filtrate X₂ [m³/h] 0.68 1.36 0.34 0.68 Filtratetemperature [° C.] 80 80 90 80 Differential pressure upon back washing-ΔP 0.19 0.40 0.10 0.18 [MPa] Other operations — — — *1 (Results ofoperations) Filtering flow rate immediately after back 0.32 0.32 0.320.31 washing of 1st cycle with filtrate [m³/h] Average filtering flowrate V₃ in 1st cycle 0.233 0.228 0.226 0.233 [m³/h] Filtering flow rateimmediately after back 0.32 0.32 0.32 0.31 washing of 2nd cycle withfiltrate [m³/h] Average filtering flow rate V₃ in 2nd cycle 0.233 0.2280.226 0.228 [m³/h] Results of operations in 3^(rd) to 5th cycles *2 *2*2 *2 Time elapsed until performance for filtering 6 8 6 3 fine crystalsbecame unable to be maintained [hrs] Comparative Examples 1 2 3 4(Operation conditions) Time of operation for filtering fine crystals T₁500 500 500 500 [s] Time of back washing operation with filtrate T₂ 7015 15 300 [s] Feeding linear velocity of filtrate upon back 2.85 0.842.85 0.84 washing operation [m/h] Back washing flow rate of filtrate X₂[m³/h] 0.68 0.20 0.68 0.20 Filtrate temperature [° C.] 80 80 70 90Differential pressure upon back washing -ΔP 0.19 0.05 0.22 0.05 [MPa](Results of operations) Filtering flow rate immediately after back 0.320.25 0.24 0.32 washing of 1st cycle with filtrate [m³/h] Averagefiltering flow rate V₃ in 1st cycle 0.145 0.247 0.233 0.088 [m³/h]Filtering flow rate immediately after back 0.32 — — 0.32 washing of 2ndcycle with filtrate [m³/h] Average filtering flow rate V₃ in 2nd cycle0.145 — — 0.088 [m³/h] Results of operations in 3rd to 5th cycles *2 — —*2 Time elapsed until performance for filtering 6 — — 1.5 fine crystalsbecame unable to be maintained [hrs] Remarks *3 *3 Note: *1: Differentvalve operations; *2: Same as those at 2nd cycle; *3: T₁ was unable tobe maintained at 2nd cycle

Example 5 Operation for Filtering Fine Crystals

The above-mentioned oxidation reaction mother liquor (80° C.) was fed tothe filtering module 7 at a circulation inlet flow rate of 0.68 [m³/h](P1=0.10 [MPaG]) (down flow) to initiate a cross-flow filtration. Atthis time, the filtering flow rate was 0.32 [m³/h] (P3=0.00 [MPaG]), andthe circulation outlet flow rate was 0.36 [m³/h] (P2=0.09 [MPaG]). Thedifferential pressure ΔP between the flow path side and the filtrationside was 0.10 [MPa]. Also, the circulating linear velocity in the flowpaths and the filtering linear velocity of the filtrate were 2848 (m/h)and 1.34 (m/h), respectively.

(Back Washing Operation with Filtrate)

When continuing the operation for filtering, the filtering flow rate wasdecreased down to 0.20 [m³/h] after the elapse of 500 seconds, andtherefore, while continuing circulation of the oxidation reaction motherliquor, the back washing operation with the filtrate was conducted for15 seconds (back washing flow rate: 0.68 [m³/h]). At this time, therespective pressures were P1=0.14 [MPaG], P2=0.10 [MPaG] and P3=0.31[MPaG], and the differential pressure −ΔP between the filtration sideand the flow path side was 0.19 [MPa].

The back washing operation with the filtrate was terminated and changedback to the operation for filtering. At this time, the filtering flowrate was 0.32 [m³/h].

(Back Washing Operation with Wash Solvent)

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated and after the elapse of 6 hours, the filteringflow rate was unable to be restored to 0.24 [m³/h] or more evenimmediately after the back washing operation with the filtrate.Therefore, while continuing circulation of the oxidation reaction motherliquor, the back washing operation with acetic acid (water content: 7.7%by mass; 85° C.) as a wash solvent was conducted for 15 seconds (backwashing flow rate: 0.68 [m³/h]). The feeding linear velocity of theacetic acid was 2.85 [m/h]. At this time, the respective pressures wereP1=0.13 [MPaG], P2=0.10 [MPaG] and P3=0.30 [MPaG], and the differentialpressure −ΔP between the filtration side and the flow path side was 0.19[MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. At this time, the filteringflow rate was restored to 0.32 [m³/h]. Next, the operation for filteringand the back washing operation with the filtrate were alternatelyrepeated again. The period was 6 hours until the filtering flow ratebecame unable to be restored to 0.24 [m³/h] or more immediately afterthe back washing operation with the filtrate, thereby conducting thenext back washing with the wash solvent. That is, while alternatelyrepeating the operation for filtering and the back washing operationwith the filtrate, the back washing operation with the wash solvent wasconducted in every six-hour period, so that the ceramic filter was ableto well maintain its filtering performance. The results are shown inTable 2.

Example 6

The operation for filtering (for 500 s) and the back washing operationwith the filtrate (for 15 seconds) were alternately and continuouslyrepeated (for 6 hours) in the same manner as in Example 5. As a result,the filtering flow rate was unable to be restored to 0.24 [m³/h] or moreimmediately after the back washing with the filtrate, and therefore,while continuing circulation of the oxidation reaction mother liquor,the back washing operation with acetic acid (water content: 7.7% bymass; 85° C.) was conducted for 15 seconds (back washing flow rate: 1.36[m³/h]). The feeding linear velocity of the acetic acid was 5.70 [m/h].At this time, the respective pressures were P1=0.15 [MPaG], P2=0.11[MPaG] and P3=0.51 [MPaG], and the differential pressure −ΔP between thefiltration side and the flow path side was 0.38 [MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. At this time, the filteringflow rate was restored to 0.32 [m³/h].

The period until conducting the next back washing with the wash solventwas 6 hours. That is, while alternately repeating the operation forfiltering and the back washing operation with the filtrate, the backwashing operation with the wash solvent was conducted in every six-hourperiod, so that the ceramic filter was able to well maintain itsfiltering performance. The results are shown in Table 2.

Example 7

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 6 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, while continuing circulation of the oxidationreaction mother liquor, the back washing operation with acetic acid(water content: 7.7% by mass; 85° C.) was conducted for 10 seconds (backwashing flow rate: 2.04 [m³/h]). The feeding linear velocity of theacetic acid was 8.54 [m/h]. At this time, the respective pressures wereP1=0.18 [MPaG], P2=0.12 [MPaG] and P3=0.69 [MPaG], and the differentialpressure −ΔP between the filtration side and the flow path side was 0.54[MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. At this time, the filteringflow rate was restored to 0.32 [m³/h]. The period until conducting thenext back washing operation with the wash solvent was 6 hours. That is,while alternately repeating the operation for filtering and the backwashing operation with the filtrate, the back washing with the washsolvent was conducted in every six-hour period, so that the ceramicfilter was able to well maintain its filtering performance. The resultsare shown in Table 2.

Example 8

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 6 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, while continuing circulation of the oxidationreaction mother liquor, the back washing operation with acetic acid(water content: 7.7% by mass; 85° C.) was conducted for 60 seconds (backwashing flow rate: 0.34 [m³/h]). The feeding linear velocity of theacetic acid was 1.42 [m/h]. At this time, the respective pressures wereP1=0.11 [MPaG], P2=0.09 [MPaG] and P3=0.22 [MPaG], and the differentialpressure −ΔP between the filtration side and the flow path side was 0.12[MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. At this time, the filteringflow rate was restored up to 0.32 [m³/h]. The period until conductingthe next back washing operation with the wash solvent was 4 hours.Furthermore, the period until conducting the further next back washingoperation with the wash solvent was also 4 hours. That is, whilealternately repeating the operation for filtering and the back washingoperation with the filtrate, the back washing operation with the washsolvent was conducted in every four-hour period, so that the ceramicfilter was able to well maintain its filtering performance. The resultsare shown in Table 2.

Example 9

The operation for filtering (for 120 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 8 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, while continuing circulation of the oxidationreaction mother liquor, the back washing operation with acetic acid(water content: 7.7% by mass; 85° C.) was conducted for 15 seconds (backwashing flow rate: 0.68 [m³/h]). The feeding linear velocity of theacetic acid was 2.85 [m/h]. At this time, the respective pressures wereP1=0.13 [MPaG], P2=0.10 [MPaG] and P3=0.30 [MPaG], and the differentialpressure −ΔP between the filtration side and the flow path side was 0.19[MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. At this time, the filteringflow rate was restored up to 0.32 [m³/h]. The period until conductingthe next back washing operation with the wash solvent was 8 hours. Thatis, while alternately repeating the operation for filtering and the backwashing operation with the filtrate, the back washing operation with thewash solvent was conducted in every eight-hour period, so that theceramic filter was able to well maintain its filtering performance. Theresults are shown in Table 2.

Example 10

The operation for filtering (for 900 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 3 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing with the filtrate, andtherefore, while continuing circulation of the oxidation reaction motherliquor, the back washing operation with acetic acid (water content: 7.7%by mass; 85° C.) was conducted for 15 seconds (back washing flow rate:0.68 [m³/h]). The feeding linear velocity of the acetic acid was 2.85[m/h]. At this time, the respective pressures were P1=0.13 [MPaG],P2=0.10 [MPaG] and P3=0.30 [MPaG], and the differential pressure −ΔPbetween the filtration side and the flow path side was 0.19 [MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. At this time, the filteringflow rate was restored to 0.32 [m³/h]. The period until conducting thenext back washing operation with the wash solvent was 3 hours. That is,while alternately repeating the operation for filtering and the backwashing operation with the filtrate, the back washing operation with thewash solvent was conducted in every three-hour period, so that theceramic filter was able to well maintain its filtering performance. Theresults are shown in Table 2.

Comparative Example 5

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 6 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, the circulation of the oxidation reactionmother liquor was terminated (i.e., the control valves 4 and 13 wereclosed, and then a drain valve 14 was opened), and then the back washingoperation with acetic acid (water content: 7.7% by mass; 85° C.) wasconducted for 180 seconds (back washing flow rate: 0.68 [m³/h]). Thefeeding linear velocity of the acetic acid was 2.85 [m/h]. At this time,the respective pressures were P1=0.00 [MPaG], P2=0.00 [MPaG] and P3=0.19[MPaG], and the differential pressure −ΔP between the filtration sideand the flow path side was 0.19 [MPa].

The back washing operation discharging the wash solvent (acetic acid)out of the system was terminated and changed back to the operation forfiltering. At this time, although the filtering flow rate was restoredup to 0.32 [m³/h], the used amount of the acetic acid therein was 0.0340[m³], i.e., as large as 12 times that used in Example 5. Further, 0.37%by weight of isophthalic acid contained in the oxidation reaction motherliquor was discharged as a waste out of the system. Meanwhile, theperiod until conducting the next back washing operation with the washsolvent was 6 hours. The results are shown in Table 2.

Comparative Example 6

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 6 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, the circulation of the oxidation reactionmother liquor was terminated (i.e., the control valves 4 and 13 wereclosed, and then the drain valve 14 was opened), and the back washingwith acetic acid (water content: 7.7% by mass; 85° C.) was conducted for15 seconds (back washing flow rate: 0.68 [m³/h]). The feeding linearvelocity of the acetic acid was 2.85 [m/h]. At this time, the respectivepressures were P1=0.00 [MPaG], P2=0.00 [MPaG] and P3=0.19 [MPaG], andthe differential pressure −ΔP between the filtration side and the flowpath side was 0.19 [MPa].

The back washing operation discharging the wash solvent (acetic acid)out of the system was terminated and changed back to the operation forfiltering. Although the amount of the acetic acid used therein was0.0028 [m³] same as that used in Example 5, it was difficult to removethe fine crystals deposited on the filtering surface on the flow pathside owing to no circulation of the mother liquor, so that the filteringflow rate was restored to only 0.26 [m³/h]. For this reason, the perioduntil conducting the 2nd or subsequent back washing operation with thewash solvent was extremely shortened to 30 min. That is, the period ofthe operation capable of maintaining a good filtering performance of theceramic filter became extremely short. The results are shown in Table 2.

Further, 6.67% by weight of isophthalic acid contained in the oxidationreaction mother liquor were discharged as a waste out of the system. Theresults are also shown in Table 2.

Example 11

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 6 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, while continuing the circulation of theoxidation reaction mother liquor, the back washing operation with aceticacid (water content: 7.7% by mass; 85° C.) as a wash solvent wasconducted for 180 seconds (back washing flow rate: 0.68 [m³/h]). Thefeeding linear velocity of the acetic acid was 2.85 [m/h]. At this time,the respective pressures were P1=0.13 [MPaG], P2=0.10 [MPaG] and P3=0.30[MPaG], and the differential pressure −ΔP between the filtration sideand the flow path side was 0.19 [MPa].

The back washing operation with the hydrous acetic acid was terminatedand changed back to the operation for filtering. At this time, althoughthe filtering flow rate was restored to 0.32 [m³/h], the amount of theacetic acid used therein was 0.034 [m³], i.e., as large as 12 times thatused in Example 5. The period until conducting the next back washingwith the wash solvent was 6 hours. The results are shown in Table 2.

Example 12

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 6 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, while continuing the circulation of theoxidation reaction mother liquor, the back washing operation with aceticacid (water content: 7.7% by mass; 85° C.) was conducted for 15 seconds(back washing flow rate: 0.14 [m³/h]). The feeding linear velocity ofthe acetic acid was 0.59 [m/h]. At this time, the respective pressureswere P1=0.10 [MPaG], P2=0.09 [MPaG] and P3=0.18 [MPaG], and thedifferential pressure −ΔP between the filtration side and the flow pathside was 0.09 [MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. The used amount of theacetic acid therein was as small as 0.00058 [m³] (i.e., 0.21 time thatused in Example 5), and the filtering flow rate was restored to only0.27 [m³/h]. For this reason, the period until conducting the 2nd orsubsequent back washing operation with the wash solvent was extremelyshortened to 1 hour. That is, the period of the operation capable ofmaintaining a good filtering performance of the ceramic filter becameextremely short. The results are shown in Table 2.

Example 13

The operation for filtering (for 500 seconds) and the back washingoperation with the filtrate (for 15 seconds) were alternately andcontinuously repeated (for 6 hours) in the same manner as in Example 5.As a result, the filtering flow rate was unable to be restored to 0.24[m³/h] or more immediately after the back washing operation with thefiltrate, and therefore, while continuing the circulation of theoxidation reaction mother liquor, the back washing operation with aceticacid (water content: 7.7% by mass; 85° C.) as a wash solvent wasconducted for 15 seconds (back washing flow rate: 5.10 [m³/h]). Thefeeding linear velocity of the acetic acid was 21.4 [m/h]. At this time,the respective pressures were P1=0.40 [MPaG], P2=0.31 [MPaG] and P3=1.41[MPaG], and the differential pressure −ΔP between the filtration sideand the flow path side was 1.06 [MPa].

The back washing operation with the acetic acid was terminated andchanged back to the operation for filtering. At this time, although thefiltering flow rate was restored to 0.32 [m³/h], the used amount of theacetic acid therein was 0.0213 [m³], i.e., as large as 7.6 times thatused in Example 5. The period until conducting the next back washingwith the wash solvent was 6 hours. The results are shown in Table 2.

TABLE 2 Examples 5 6 7 8 9 10 (Operation conditions in 1st cycle) Timeof operation for filtering 500 500 500 500 120 900 fine crystals [s]Time of back washing operation 15 15 15 15 15 15 with filtrate [s] Timeof back washing operation 15 15 10 60 15 15 with wash solvent [s]Circulation of oxidation reaction Done Done Done Done Done Done motherliquor upon back washing operation with wash solvent Feeding linearvelocity of wash 2.85 5.70 8.54 1.42 2.85 2.85 solvent [m/h]Differential pressure upon back 0.19 0.38 0.54 0.12 0.19 0.19 washingwith wash solvent [MPa] (Results of operations at 1st cycle) Period ofback washing operation 6 6 6 6 8 3 with wash solvent [hrs] Filteringflow rate immediately 0.32 0.32 0.32 0.32 0.32 0.32 after back washingoperation with wash solvent [m³/h] Amount of wash solvent used 0.00280.0057 0.0057 0.0057 0.0028 0.0028 [m³] Amount of isophthalic acid 0 0 00 0 0 discarded [%] (Results of operations in 2nd or subsequent cycle)Period of back washing operation 6 6 6 4 8 3 with wash solvent [hrs]Comparative Examples Examples 5 6 11 12 13 (Operation conditions at 1stcycle) Time of operation for filtering 500 500 500 500 500 fine crystals[s] Time of back washing operation 15 15 15 15 15 with filtrate [s] Timeof back washing operation 180 15 180 15 15 with wash solvent [s]Circulation of oxidation reaction Not Not Done Done Done mother liquorupon back washing done done operation with wash solvent Feeding linearvelocity of wash 2.85 2.85 2.85 0.59 21.4 solvent [m/h] Differentialpressure upon back 0.19 0.19 0.19 0.09 1.06 washing with wash solvent[MPa] (Results of operations at 1st cycle) Period of back washingoperation 6 6 6 6 6 with wash solvent [hrs] Filtering flow rateimmediately 0.32 0.26 0.32 0.27 0.32 after back washing operation withwash solvent [m³/h] Amount of wash solvent used 0.0340 0.0028 0.03400.00058 0.0213 [m³] Amount of isophthalic acid 0.37 6.67 0 0 0 discarded[%] (Results of operations in 2nd or subsequent cycle) Period of backwashing operation 6 0.5 6 1 6 with wash solvent [hrs]

INDUSTRIAL APPLICABILITY

In the filtering operation method according to the present invention,there can be attained such advantages that (1) a filtering operation canbe simply performed for a long period of time without clogging of aceramic filter; (2) no use of a large amount of a wash solvent isneeded; and (3) an aromatic carboxylic acid contained in the washsolvent which has been conventionally discarded together with the washsolvent upon cleaning can be recovered. Therefore, it is possible tosimplify the operation conditions and improve an output level of anaromatic carboxylic acid upon production of the aromatic carboxylicacid.

EXPLANATION OF REFERENCE NUMERALS

The reference numerals used in FIG. 1, FIG. 2 and FIG. 3 are explainedbelow.

-   -   1: Oxidation reaction mother liquor vessel; 2: Pump; 3: Minimum        flow line; 4: Control valve; 5: Pressure gauge (P1); 6:        Circulation inlet conduit; 7: Filtering module (housing); 8:        Ceramic filter; 9: Filtration side; 10: Substrate end surface        side; 11: Circulation outlet conduit; 12: Pressure gauge (P2);        13: Control valve; 14: Drain valve; 15: Circulation return        conduit; 16: Filtrate outlet conduit; 17: Pressure gauge (P3);        18: Valve; 19: Filtrate vessel; 20: Pump; 21: Minimum flow line;        22: Heat exchanger; 23: Filtrate back washing conduit; 24:        Control valve; 25: Control valve; 26: Wash solvent back washing        conduit; 27: Heat exchanger; 28: Flow path

The invention claimed is:
 1. A filtering operation method for filteringfine crystals contained in an oxidation reaction mother liquor obtainedin a process for producing an aromatic carboxylic acid except forterephthalic acid by a cross-flow filtration using a ceramic filterwhile conducting a flowing circulation operation of the oxidationreaction mother liquor, the method comprising: conducting (I) anoperation for filtering the fine crystals; and conducting (II) a backwashing operation with a filtrate while maintaining the flowingcirculation operation of the oxidation reaction mother liquor, wherein(II) the back washing operation with the filtrate is conducted under thefollowing conditions: (II-A) an operation time that is in the range offrom 5 to 60 s; (II-B) a differential pressure between a filtration sideand a flow path side of the ceramic filter which is in the range of from0.10 to 1.0 MPa; (II-C) a feeding linear velocity of the filtrate thatis in the range of from 1.0 to 20 m/h; and (II-D) a temperature of thefiltrate that is in the range of from a temperature of the oxidationreaction mother liquor to a temperature higher by 35° C. than that ofthe oxidation reaction mother liquor.
 2. The filtering operation methodaccording to claim 1, wherein (I) the operation for filtering the finecrystals is conducted under the following conditions: (I-A) an operationtime thereof that is in the range of from 60 to 1800 s; (I-B) adifferential pressure between the flow path side and the filtration sidethat is in the range of from 0.05 to 0.5 MPa; (I-C) a circulating linearvelocity of the oxidation reaction mother liquor in the flow path of theceramic filter that is in the range of from 1000 to 10000 m/h asmeasured at an inlet of the flow path; and (I-D) a filtering linearvelocity of the filtrate that is in the range of from 0.5 to 3.0 m/h. 3.The filtering operation method according to claim 1, further comprising:conducting (III) a back washing operation with a wash solvent whilemaintaining the flowing circulation operation of the oxidation reactionmother liquor.
 4. The filtering operation method according to claim 3,wherein (III) the back washing operation with the wash solvent isconducted under the following conditions: (III-A) an operation timethereof that is in the range of from 5 to 120 s; (III-B) a differentialpressure between the filtration side and the flow path side upon theback washing operation with the wash solvent that is in the range offrom 0.10 to 1.0 MPa; (III-C) a feeding linear velocity of the washsolvent upon the back washing operation that is in the range of from 1.0to 20 MPa; and (III-D) a temperature of the wash solvent that is in therange of from the temperature of the oxidation reaction mother liquor tothe temperature higher by 35° C. than that of the oxidation reactionmother liquor.
 5. The filtering operation method according to claim 3,wherein an operation comprising (I) the operation for filtering the finecrystals and (II) the back washing operation with the filtrate isrepeated, and when a flow rate of the filtering operation is notrestored by (II) the back washing operation with the filtrate, (III) theback washing operation with the wash solvent is conducted.
 6. Thefiltering operation method according to claim 3, wherein the washsolvent is acetic acid having a water content of from 0.1 to 30% bymass.
 7. The filtering operation method according to claim 1, whereinupon the back washing operation with the filtrate, a pressure on acirculation outlet conduit side of the ceramic filter is reduced toproduce a differential pressure, thereby feeding the filtrate.
 8. Thefiltering operation method according to claim 1, wherein the aromaticcarboxylic acid comprises at least one of benzoic acid, phthalic acid,isophthalic acid, m-toluic acid, trimesic acid, 3,5-dimethyl benzoicacid, trimellitic acid, pyromellitic acid, 1,5-naphthalenedicarboxylicacid and 2,6-naphthalenedicarboxylic acid.
 9. The filtering operationmethod according to claim 1, wherein the filtering operation and theback washing operation are alternately repeated.
 10. A method forfiltering fine crystals obtained from aromatic carboxylic acidproduction other than terephthalic acid production, the methodcomprising: maintaining a flowing circulation of an oxidation reactionmother liquor obtained in a aromatic carboxylic acid production otherthan terephthalic acid production; filtering the fine crystals during afiltering operation and utilizing cross-flow filtration with a ceramicfilter; and back washing with a filtrate during a backwashing operation,wherein the back washing operation occurs while maintaining the flowingcirculation operation of the oxidation reaction mother liquor and underthe following conditions: an operation time of from 5 to 60 s; adifferential pressure between a filtration side and a flow path sidethat is from 0.10 to 1.0 MPa; a feeding linear velocity of the filtratethat is from 1.0 to 20 m/h; and a temperature of the filtrate that isbetween temperature A and temperature B with temperature A being atemperature of the oxidation reaction mother liquor and temperature Bbeing a temperature higher by 35° C. than that of the oxidation reactionmother liquor.
 11. The filtering operation method according to claim 10,wherein the filtering operation is conducted under the followingconditions: an operation time that is in the range of from 60 to 1800 s;a differential pressure between the flow path side and the filtrationside that is in the range of from 0.05 to 0.5 MPa; a circulating linearvelocity of the oxidation reaction mother liquor in the flow path of theceramic filter that is in the range of from 1000 to 10000 m/h at aninlet of the flow path; and a filtering linear velocity of the filtratethat is in the range of from 0.5 to 3.0 m/h.
 12. The filtering operationmethod according to claim 10, wherein the filtering operation and theback washing operation are alternately repeated.